The company has improved the performance of the anti-malware engine, including its clean-up capabilities, wrote Brandon LeBlanc, a Windows communication manager at Microsoft. Security Essentials was released in September 2009 and replaced Windows Live OneCare, which the company no longer sells.

Security Essentials will also ask whether a user wants to turn on the firewall. Another improvement is integration with the company's Internet Explorer browser, which LeBlanc said provides protection against Web-based threats.

The software will also now detect network-based threats, but only for the Vista and Windows 7 OSes. It will not work on XP because that OS does not have the Windows Filtering Platform, a set of APIs (application programming interfaces) that let software vendors filter and modify TCP/IP packets for configuring firewalls or other packet processing.

The limited beta is available in English for the U.S. and Israel, and in Brazilian Portuguese. It will also be available in simplified Chinese later this year, LeBlanc wrote. It will be available on a first-come, first-served basis until all the slots for the beta are taken, he wrote.

LeBlanc wrote on Tuesday that Microsoft was having temporary problems with the Microsoft Connect download but offers an alternate way to download the software.